1. Field of the Invention
This invention relates generally to a means for cylindrically diffusing energy from an optical wave guide, and more particularly, to a cylindrical diffuser tip of an optical fiber useful for performing Photodynamic Therapy in the treatment of diseased tissue such as tumors, inducing hyperthermia or performing both percutaneous and intraoperative phototherapy of cardiovascular disease.
2. Reference to Co-Pending Patent Application
Reference is made to co-pending patent application U.S. Ser. No. 07/608,006 filed Nov. 1, 1990 entitled Diffusion Tip for Optical Fibers by two of the present inventors and having a common assignee with the present application.
3. Description of the Prior Art
Photodynamic treatment of tumors using hematoporphyrin derivatives requires that a tumor under treatment be irradiated with light usually around, but not limited to, a wavelength of 630 nanometers generally from a laser. A short time prior to irradiation, the patient is injected with a photo-sensitive compound which accumulates in the vascular stoma of the tumor and in cells. Subcutaneous tumors greater than 0.5 cm thick, also referred to herein as interstitial tumors, undergoing this treatment require the use of optical fibers to guide the light from the source to the treatment area. In many cases, the outlet termination of the fiber is inserted directly into the tumor. In other cases, where the tumor is located in passages, for example endobronchial tumors, the optical fiber termination is positioned intraluminally in close proximity to the tumor. Efforts have been directed in recent years to developing suitable fiber terminations for the delivery of a uniform, predictable dosage of effective irradiation of light to a large volume of tumor tissue.
Considerable light radiation must be transmitted to kill large tumor masses by photo irradiation and the required radiation can cause overheating, especially if it is concentrated in too small a region. This causes problems in delivering radiant energy out of the end of a normal blunt ended or flat cut optical fiber, making a small hot spot which may lead to excessive heating, carbonization and necrosis of the adjacent tissue making it opaque to transillumination. Thermal sources such as xenon arc lamps also pose difficulties in transmitting adequate radiation to deep-seated tumors because non-coherent sources cannot be coupled efficiently to reasonably small optical fibers for delivery to the tumor. Problems of distributing radiation uniformly throughout the region of a tumor to be killed are also formidable.
There are several ways that the cylindrical diffusion of radiant energy from an optical fiber core can be accomplished. One way is to choose a ratio of the indices of refraction between the outer cladding and the core region of the optical fiber so that internal reflection within the core region is substantially less than total. This causes light to enter the cladding- If scattering centers are present in the cladding the light can radiate outward to emerge through the (preferably transparent) cladding.
Another way is to alter the interface between the fiber optic core and cladding to increase side radiation. Texturing the outer surface of the core region to provide a ground glass effect is one method commonly used. Another is positioning or embedding light scattering elements such as tiny particles at the surface of the fiber optic core near the interface with the cladding. Light scattering particles can also be imbedded throughout the cladding to enhance the side delivery of radiation. Combinations of these measures are also possible.
For example, Chapman in United Kingdom patent GB2154761A (issued Sep. 11, 1985), which is incorporated herein by reference, describes an optical fiber for use in Photodynamic Therapy wherein the fiber comprises a central core material enveloped by a special two-layer cladding. The cladding comprises an inner cladding of a low refractive index material and an outer cladding. The fiber, being adapted to be coupled to a laser beam, has an output end portion which has a tapered core region which is surrounded by a diffusing medium. Light emerging from the tapered core region undergoes scattering.
In one preferred embodiment, Chapman's core is of circular cross section and the diameter of the core in the tapered region decreases uniformly to an end most point over a length of between 5 and 15 millimeters. In a further preferred embodiment, Chapman describes a diffusion medium comprising a transparent resin material, which contains fine particulate reflective or refractive matter.
Clark, in U.S. Pat. No. 4,336,809 (issued Jun. 29, 1982) describes a tissue photo irradiation system for use with hematoporphyrin dyes and derivatives thereof. In Clark's system he describes the use of an optical needle which serves as a linear radiator or a cylindrical diffuser and which can be coupled to an optical fiber by means of a conventional optical coupler. Clark's needle includes a fiber optic core that is generally internally reflecting. The core is surrounded by a cladding as generally known; but in an end region a different cladding surrounds the core to make it into a radiator instead of an internally reflecting transmitter. When the cladding contains scatterers, the "needle" or diffusion tip comprises a transparent core surrounded by a scattering layer in which the concentration of scatterers is homogeneous along its length.
Production of a controllable level of temperature elevation or hyperthermia at pre-selected locations in volumes of tissue has been found to be of significant therapeutic value in the treatment of patients with cancer. In particular, hyperthermia may, in some cases, have a synergistic effect when used in conjunction with Photodynamic Therapy for treating tumors or performing angioplasty. At the high power levels required for hyperthermia or hyperthermia plus Photodynamic Therapy, high peak intensities or hot spots can lead to excessively high temperatures causing unintentional non-selective tissue damage. It is, therefore, desirable to distribute the illuminating energy evenly within the target volume to achieve uniform temperature distributions.
The present fiber optic cylindrical diffuser tip technology is limited in clinical applications due to the following:
a) The underlying fiber optic is weakened by mechanical processing during manufacturing of the cylindrical diffusing tip;
b) The weakened fiber optic limits the flexibility of the finished cylinder diffuser to the point of sole quasi-rigid usage (very limited endoscopic use);
c) Output sensitivity of prior art cylindrical diffuser tips to input beam convergence causes extreme variability in the output intensity distribution.
d) A non-uniform output intensity distribution makes treatment dosimetry uncertain and clinical results inconsistent; It is desirable, therefore, to provide a cylindrical diffuser for use as a termination on an optical fiber which overcomes most or all of the limitations stated above.
McCaughan, Jr., in U.S. Pat. No. 4,660,925 (Issued Apr. 28, 1987) incorporated herein by reference, describes a cylindrical diffuser tip that overcomes some of the problems with prior art diffuser tips. McCaughan Jr. suggests (column 4, lines 48-62) providing a tip surrounding the core of an optical fiber, the tip containing a gradient of scatterers which increases logarithmically in concentration along the fiber axis in a direction toward the polished tip of the optical fiber. To accomplish this, McCaughan, Jr. teaches a method for making such a tip comprising the steps of exposing the core of an optical fiber near its tip, polishing the exposed core and repeatedly dipping the tip in a medium containing different concentrations of scatterers to allegedly increase the concentration of scatterer along the length of the exposed core. The polished tip of the core (column 5, lines 47-49) region is cleaned of scattering medium upon removal from the dipping vessel. The word "allegedly" is used above because such a method of repetitive coating followed by the step underlined above is inoperable to produce a longitudinal gradient of scatterer in a diffuser tip. This method produces a radial gradient in scatterer concentration which varies radially with distance from the fiber core axis. Even if this method could, by further experimentation, be made operable, such a method would provide, at best, a discrete, step-wise concentration gradient which would only approximate a logarithmic gradient in the limit of infinite coatings.
Two of the present inventors (D. D., H. N., Jr.), in co-pending application U.S. Ser. No. 07/608,006 filed Nov. 1, 1990 entitled: Diffusion Tip for Optical Fibers, suggest a composite tip comprised of laminated layers or "plugs" of elastomer, each plug having a higher concentration of scatterer embedded therein than the preceding layer as one moves away from the optical fiber tip along the fiber axis. While such a tip is an improvement over McCaughan, Jr. in that it provides a step-wise concentration gradient in a longitudinal direction but not a radial direction as with McCaughan (i.e.: no dipping of the exposed core is involved), and only approximates a continuous concentration gradient but does not produce a gradient that is optimal.